Extensions

Use extensions for MCP methods that are not one of the built-in tools, resources, prompts, or transport operations. Extension names must be fully qualified, for example example.com/telemetry.

Register an extension

import std/[asyncdispatch, json]
import nimwire

let server = newMcpServer("extensions", "1.0.0")
let extension = newMcpExtension(
  "example.com/echo",
  capabilities = %*{"enabled": true},
  metadata = %*{"example.com/echo": {"version": 1}})

extension.addExtensionMethod(
  "example/echo",
  proc (params: JsonNode,
        context: McpContext): Future[McpWireResult] {.async.} =
    newMcpResult(mcpComplete, %*{"value": params["value"]}),
  inputSchema = %*{
    "type": "object",
    "properties": {"value": {"type": "string"}},
    "required": ["value"],
    "additionalProperties": false
  })

server.registerExtension(extension)
server.finalizeExtension("example.com/echo")

Only finalized extensions are advertised in discovery and dispatched. Finalize after all methods and schemas have been added.

Client capabilities

Set requiresClientCapability = true when the extension must be explicitly supported by the client:

let extension = newMcpExtension(
  "example.com/approval",
  requiresClientCapability = true)

Calls fail with mcpMissingRequiredClientCapabilityCode until the request metadata contains that extension capability. This is also how the built-in Tasks extension protects its methods.

inputSchema and outputSchema are validated at registration and dispatch boundaries. transportRules and method metadata are retained for the extension’s wire contract.

Related: Tasks and the extensions API reference.